1. Field of the Invention
The present invention relates to an isolator circuit which isolates a reference potential from another reference potential on the side of an input signal.
2. Description of the Related Art
An isolator circuit of this kind is known as a circuit constituting a part of a balanced amplifying circuit, as disclosed in, for example, JP-A-10-082439. FIG. 3 shows a conventional isolator circuit configured by an operational amplifier and resistors R. In this configuration, when input signals Vin(+) and Vin(xe2x88x92) are input in balance to the non-inverting input terminal (+) and the inverting input terminal (xe2x88x92) of the operational amplifier through the resistors R, respectively, a signal corresponding to the voltage difference between the signals is output from an output terminal of the operational amplifier. As a result, common mode noises contained in the input signals Vin(+) and Vin(xe2x88x92) can be eliminated therefrom.
In such an isolator circuit, in order to lower the noise voltage, resistors of a low resistance are usually used as the resistors R. When the resistors R have a low resistance, the operational amplifier must be provided with a large load driving capability, and hence the input impedance is low. Therefore, buffer circuits are connected through the resistors R to the non-inverting input terminal (+) and the inverting input terminal (xe2x88x92) of the operational amplifier, respectively.
In the conventional isolator circuit, the connection of the buffer circuits to the non-inverting input terminal (+) and the inverting input terminal (xe2x88x92) of the operational amplifier increases the current consumption and enlarges the circuit scale.
As a result, when the conventional isolator circuit is incorporated into, for example, an IC, there arise problems in that the current consumption of the IC is increased, and that the occupied area of the isolator circuit in the IC is increased to impede the miniaturization of the IC.
It is an object of the invention to provide an isolator circuit which solves the problems, and consumes a small amount of a current, and in which the circuit scale is small.
In order to attain the object, according to a first aspect of the invention, an isolator circuit comprises: a first differential pair circuit which compares voltages of two input signals with each other; resistors which are connected to two output terminals of the first differential pair circuit, respectively; a current comparison circuit which compares currents respectively flowing through the two resistors to output a voltage corresponding to a result of the comparison; and a second differential pair circuit which compares the voltage output from the current comparison circuit with a reference voltage, and two output terminals of the second differential pair circuit are respectively connected to the two resistors to equalize the currents flowing through the resistors.
In the thus configured invention, the first differential pair circuit compares voltages of the two input signals with each other, and, in accordance with the ratio of the voltages, currents flow through the resistors respectively connected to the two output terminals of the first differential pair circuit. The current comparison circuit compares the currents respectively flowing through the two resistors, and outputs a voltage corresponding to a result of the comparison. The second differential pair circuit compares the voltage output from the current comparison circuit with the reference voltage, and a negative feedback is conducted so that the currents respectively flowing through the two resistors are equal to each other. Unlike the conventional art, therefore, an isolator circuit can be configured without disposing buffer circuits, and hence the current consumption is small, and the circuit scale can be made smaller. Furthermore, the common-mode rejection ratio can be set to be high.
According to a second aspect of the invention, an isolator circuit comprises: a first differential pair circuit having first and second active elements; a second differential pair circuit having third and fourth active elements; two resistors; and a current comparison circuit, a control terminal of the first active element is connected to one input terminal for a balanced input signal, an output terminal of the first active element is connected to one input terminal of the current comparison circuit, and also to one of the resistors, a control terminal of the second active element is connected to another input terminal for the balanced input signal, an output terminal of the second active element is connected to another input terminal of the current comparison circuit, and also to another one of the resistors, a control terminal of the third active element is AC-grounded, an output terminal of the third active element is connected to the output terminal of the first active element, a control terminal of the fourth active element is connected to an output terminal of the current comparison circuit, and an output terminal of the fourth active element is connected to the output terminal of the second active element.
In the thus configured invention, the first differential pair circuit compares one voltage of the balanced input signal which is applied from the one input terminal for the balanced input signal to the control terminal of the first active element, with the other voltage of the balanced input signal which is applied from the other input terminal for the balanced input signal to the control terminal of the second active element, and currents corresponding to a result of the comparison flow through the two resistors, respectively. The current comparison circuit compares the currents with each other, and outputs a voltage corresponding to a result of the comparison. The output voltage is applied to the control terminal of the fourth active element, and a reference voltage is applied to the control terminal of the third active element. The second differential pair circuit compares the voltages with each other. On the basis of a result of the comparison, a negative feedback is conducted so that the currents respectively flowing through the two resistors are equal to each other. Unlike the conventional art, an isolator circuit can be configured without disposing buffer circuits. Therefore, the current consumption is small, and the circuit scale can be made smaller. Furthermore, the common-mode rejection ratio can be set to be high.
A third aspect of the invention is configured so that, in the isolator circuit of the second aspect of the invention, the first to fourth active elements are transistors.
A fourth aspect of the invention is configured so that, in the isolator circuit of the second or third aspect of the invention, a constant voltage is applied to the control terminal of the second active element. Therefore, the constant voltage applied to the control terminal of the third active element can be used as the reference voltage which is to be compared with the output voltage.